Fluoropolymers are generally made by an aqueous dispersion process, which provides a suitable heat sink for controlling the heat of polymerization and can produce a high yield and high molecular weight relative to polymerization conducted in an organic solvent. In order to achieve stable dispersion or emulsion, a suitable surfactant or emulsifier must be employed. Fluorinated-surfactants are generally used because they can yield stable particle and high molecular weight fluoropolymers. However, the fluorinated-surfactants typically used in emulsion polymerization of fluoropolymers, such as the ammonium salt of perfluoro octanoic acid or salts of perfluoro sulfonic acids are expensive. They also present an environmental concern related to bio-persistence.
It is therefore desirable to carry out an emulsion polymerization of fluoropolymers in the absence of fluorinated-surfactants, without compromising the properties of the resultant fluoropolymers. It would also be desirable to produce small particle size emulsion so that latex stability during storage as well as quality of film formation is improved. Furthermore, it would be desirable to produce a latex and/or fluoropolymer having fewer extractable ions and extractable low molecular weight polymers, while generally yielding fluoropolymers that have similar or even improved properties compared to analogous fluoropolymers made in presence of added fluorinated-surfactants.
Emulsion polymerization of vinylidene fluoride at moderate pressures and temperatures using fluorinated surfactants, free radical initiators, and trichlorofluoromethane as chain transfer agent is taught in the U.S. Pat. No. 4,569,978 in which VF2 based polymers are produced with reduced tendency to generate cavity and greater resistance to discoloration at elevated temperatures. The process was refined in the U.S. Pat. No. 6,734,264 wherein particularly ozone depleting agent (trichlorofluoromethane) was replace by propane which is environmentally friendly chemical. It is noteworthy that in both processes fluorinated surfactant was needed to produce stable emulsion. For example, perfluorocarboxylate salts was used to stabilize fluoropolymer emulsion polymerizations, with the most common example being ammonium perfluorooctanoate or ammonium perfluoronanoate. The high degree of fluorination is thought to be necessary to prevent chain transfer reaction between a growing polymer chain and the surfactant which in turn may result in lowering molecular weight and/or inhibition of the polymerization.
Many attempts have been made to find a suitable emulsifier in place of fluorinated surfactant for such polymerizations, as disclosed in the background section of U.S. Pat. No. 6,512,063 in which sodium salt of hydrocarbon sulfonates as non-fluorinated but ionic emulsifier was employed. The ionic emulsifiers are undesired for high purity applications due to high levels extractable ions. Furthermore, hydrocarbon sulfonates act as implicit chain transfer agent in the emulsion polymerization of fluoropolymers, as a result, it cannot be used in sufficient amount to produce small particle size latex without inhibiting such polymerizations.
Emulsifier-free aqueous emulsion polymerization process for making fluoropolymer such as TFE and/or VDF copolymers is described in U.S. Pat. No. 6,693,152. In emulsifier free emulsion polymerization, first only inorganic ionic initiators such as persulfates or permangamates may work whereas organic peroxide initiators would not work. Second, the particle size of emulsifier free emulsion of fluoropolymers would be large; as a result, the shelf-life of latex would be very limited. Third, the solid content of emulsifier free latex is limited to low or moderate solids, where in fact a high solid latex is desirable in variety of commercial applications.
Surprisingly it has now been found that a fluoropolymer can be made by a process using non-fluorinated, non-ionic emulsifiers containing segments of polyethylene glycol and/or polypropylene glycol having varieties of different terminal groups and functions. The fluoropolymer dispersions produced have good latex stability and shelf-life, and are coagulum and adhesion free.